g 2 8 Theory of Cell-formation [BockII. 



bears. Meanwhile Nageli had also distinguished the protoplasm 

 from everything else in the cell, and noticed its pre-eminent 

 importance in cell-formation and its nitrogenous character. 



We must not omit to mention here, that investigations into 

 the processes of cell-formation compelled observers to search 

 for the spots where cell-formation actually takes place, and 

 thus the fact was ascertained, that cells in statu nascendi are 

 not to be found in all parts, not even in all growing parts of 

 the plant, but that we must look for them in the so-called 

 puncta vegetationis in the stem and root, in the youngest 

 lateral organs, and between the bark and the wood in woody 

 plants. About this time a new idea began to be attached to 

 the word cambium, which Mirbel had used in the sense of a 

 nourishing juice saturating the plant ; it was now applied to 

 the tissue-masses in which the formation of new cells takes 

 place, and specially to the very thin layer of tissue lying 

 between the wood and the rind, from which new layers of 

 wood and rind in woody plants are formed— a layer, which 

 according to Mirbel's theory had been a mass of sappy matter, 

 in which new cells arise as vacuoles. 



Unger in an enquiry into the growth of internodes (' Bota- 

 nische Zeitung,' 1844) again declared himself as an opponent of 

 Schleiden's theory. He maintained first of all and erroneously 

 that the cell-nucleus is not of general occurrence in tissue where 

 division is taking place, but he argued rightly from the position 

 of the cells, from the difference of thickness in their walls, and 

 from their relative size, in favour of their multiplication by 

 the formation of dividing walls ; he noticed the part played by 

 the cell-contents in the multiplication of cells in hairs, and 

 asserted that merismatic cell-formation (cell-division) is the 

 general rule in the growth of organs of vegetation, while he 

 distinctly declared that it was not possible to bring ail that is 

 actually seen at the spots where formation of cellular tissue is 

 taking place into agreement with Schleiden's theory. But 

 Unger did not observe the processes that take place in cell- 



